Dictionary.net

A deduction allowed by the federal estate tax laws for all property passed to a surviving spouse who is a U.S. citizen. This deduction (which really functions as an exemption) allows anyone, even a billionaire, to pass his or her entire estate to a surviving spouse without any tax at all.

The tax-free transferal to a spouse of a specified asset by a will. Current tax laws exclude all federal estate tax from an unlimited transferred amount.